Nuclear reactor core loading and operation strategies

ABSTRACT

Cores include different types of control cells in different numbers and positions. A periphery of the core just inside the perimeter may have higher reactivity fuel in outer control cells, and lower reactivity cells may be placed in an inner core inside the inner ring. Cores can include about half fresh fuel positioned in higher proportions in the inner ring and away from inner control cells. Cores are compatible with multiple core control cell setups, including BWRs, ESBWRs, ABWRs, etc. Cores can be loaded during conventional outages. Cores can be operated with control elements in only the inner ring control cells for reactivity adjustment. Control elements in outer control cells need be moved only at sequence exchanges. Near end of cycle, reactivity in the core may be controlled with inner control cells alone, and control elements in outer control cells can be fully withdrawn.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 120 to, and is acontinuation of, co-pending application Ser. No. 15/444,294, filed Feb.27, 2017, which is a division of U.S. application Ser. No. 13/531,514,filed Jun. 23, 2012, now U.S. Pat. No. 9,583,223. The original contentsof these applications are incorporated by reference herein in theirentireties.

BACKGROUND

FIG. 1 is an illustration of several related art nuclear fuel bundles 10and core components commonly encountered in existing nuclear powertechnology. As shown in FIG. 1, one or more fuel bundles 10 containingseveral individual fuel rods may be placed within a reactor core inconventional fuel placement strategies. A channel 20 may surround thefuel rods in each bundle 10, providing directed coolant and/or moderatorflow within bundles 10 and/or facilitating manipulation of bundles 10 asa single rigid body. Control rods or cruciform control blades 60 may beextended from set core locations between bundles to absorb neutrons andcontrol reactivity and ultimately control reactivity by a degree ofinsertion or withdrawal from between the fuel bundles 10. Fuel support70 may support and align bundles 10 at constant positions within thecore.

FIG. 2 is a quadrant map of a related art Boiling Water Reactor (BWR)core, illustrating fuel bundle locations within one quarter of the core.Reactor cores typically are conveniently symmetrical about at least twoperpendicular axes, such that a quadrant map of FIG. 2 conveys themakeup of the entire core. As shown in FIG. 2, individual bundlelocations are occupied by fresh (shown with diagonal or cross-hatchedfill) or burnt (shown with no fill) fuel bundles at the start of a fuelcycle, before commencement of power operations in the core. Freshbundles are bundles that have not previously been exposed to neutronflux during power operations, i.e., never been burnt, whereas burnt fuelbundles have received such exposure, typically over one or more fuelcycles lasting 1-2 years. As such, burnt fuel bundles typically haveexposure, or burnup, of several GWd/ST.

Fresh fuel bundles may have different starting enrichments of fissilematerial content. For example, in some BWR designs, outer-enrichmentbundles (shown in cross-hatched fill) may include approximately 4.3%Uranium-235 fuel, and inner-enrichment bundles (shown in diagonal fill)may include approximately 4.2% Uranium-235 fuel. Varying enrichments,such as the one shown in FIG. 2, may permit a flatter radial powerprofile in the core and/or achieve other operational effects. Further,in some BWR designs, bundles may also possess varying distributions andconcentrations of burnable poisons/neutron absorbers to suppressreactivity and optimize operational characteristics. As shown in FIG. 2,at startup related art nuclear fuel cores include an outer peripheralring of stale fuel bundles surrounding an inner peripheral ring offresh, high-enrichment fuel bundles. A central region may include 50% ormore fresh bundles in order to maximize fresh fuel content over an evendistribution, permitting longer operating cycles with lower downtime.

In related art BWRs, cruciform control blades 60 extend centrallybetween groupings of four fuel bundles in order to absorb neutrons andcontrol the nuclear chain reaction in the core. As shown in FIG. 2, thegroupings of four fuel bundles, between which control blades extend, areidentified in bolded outline as controlled bundles, or control cells.Bundles within the controlled bundle groups conventionally have one faceclosest to a control blade used during the fuel cycle; such bundles arereferred to as controlled bundles and their positions as controlledpositions in control cells of four bundles. Different control blades indifferent control cells, usually four or five per quadrant, areconventionally alternately inserted and withdrawn in different andcomplex control blade sequences in order to manage reactivity and powerdistribution and spread control blade usage across several differentblades and fresh fuel bundles within the core.

As shown in FIG. 2, in order to maximize the number of fresh fuelbundles used in a longer cycle over an even core distribution, severalfresh fuel bundles may be placed in controlled positions adjacent tooperated control blades within the inner portion of the core. Due toconventional operation of control blades, all fresh fuel bundles in thecentral core portion may be controlled—having direct exposure to controlblades actively moved to finely control reactivity—throughout an entirefuel cycle. Use of fresh fuel bundles in controlled locations causesseveral problems, including corrosion and channel bowing that worsens inlater cycles, and a need to perform complex and/or lower-power controlblade sequence exchanges due to this positioning that worsen planteconomics. Some related art fuel cores have avoided this problem byusing a Control Cell Core loading strategy, where only burnt fuelbundles are placed closest to operated control blades, resulting infewer fresh bundles used in the central portion of the core and shorteroperating cycles.

SUMMARY

Example embodiments include nuclear cores with at least two control celltypes that differ in total reactivity. The different control cell typesmay be placed in numbers and/or positions the enhance fuel and coreperformance. Example cores may include an outermost region with lowerreactivity fuel bundles, an inner peripheral region lining the outerperipheral region and having higher reactivity fuel bundles and at leastportions of the outermost control cells, and an inner core lining theinner peripheral region and having inner control cells with only fuelbundles of lower reactivity. The lower reactivity bundles may be burnt,and the higher reactivity bundles may be fresh, for example, the outercontrol cells can include two fresh fuel bundles and the inner controlcells can include only burnt fuel bundles. However, reactivitydifferences may also be achieved through fuel enrichment variation,burnable poison presence, etc. In an example with a conventional BWR,the inner peripheral region may be three bundles thick, most of whichcan be higher reactivity fuel bundles, and the outer peripheral regionmay be three bundles thick. In this instance, there may be thirteeninner control cells. Example embodiments are not limited to BWRs orspecific placements, but are compatible with any type of core controlcell setup, including control cells formed with control rods orcruciform control blades having four fuel bundles positioned in eachcorner the blades. Different core geometries are easily outfitted withexample embodiments; for example, in an ESBWR, the inner core region mayhave twenty-five inner control cells.

Example methods include creating and/or operating nuclear cores withmultiple types of control cells. For example, a core may be loaded toform an example embodiment core. In example methods, control elements inonly the inner control cells may be moved to control core reactivity,except at sequence exchanges after several weeks or months of operation,such as after 3 GWd/ST. At such a sequence exchange, a single coarsemovement of control elements in the outer control cells may be made inorder to resume controlling day-to-day reactivity with the inner controlcells. Near the end of a cycle, when reactivity is lowest, reactivity inthe core may be controlled only with inner control cells, and controlelements in the outer control cells can be fully withdrawn.

Example embodiments and methods can provide high (approximately 50%)fresh fuel volumes for each cycle, enabling longer cycles and betterplant economics. Example methods and embodiments further provide highpower density and low leakage through segregating fuel types byreactivity in the periphery and inner portions of the core. Examplemethods and embodiments further may enable simplified andnon-interrupting movement of control elements in the inner core to fullycontrol reactivity without causing negative control element and fuelinteractions.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Example embodiments will become more apparent by describing, in detail,the attached drawings, wherein like elements are represented by likereference numerals, which are given by way of illustration only and thusdo not limit the terms which they depict.

FIG. 1 is an illustration of related art fuel bundles loaded into a corehaving cruciform blades for control elements.

FIG. 2 is a quadrant map of a related art commercial nuclear reactorcore.

FIG. 3 is a quadrant map of an example embodiment nuclear core.

FIG. 4 is a quadrant map of another example embodiment nuclear core.

DETAILED DESCRIPTION

This is a patent document, and general broad rules of constructionshould be applied when reading and understanding it. Everythingdescribed and shown in this document is an example of subject matterfalling within the scope of the appended claims. Any specific structuraland functional details disclosed herein are merely for purposes ofdescribing how to make and use example embodiments or methods. Severaldifferent embodiments not specifically disclosed herein fall within theclaim scope; as such, the claims may be embodied in many alternate formsand should not be construed as limited to only example embodiments setforth herein.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element is referred to as being“connected,” “coupled,” “mated,” “attached,” or “fixed” to anotherelement, it can be directly connected or coupled to the other element orintervening elements may be present. In contrast, when an element isreferred to as being “directly connected” or “directly coupled” toanother element, there are no intervening elements present. Other wordsused to describe the relationship between elements should be interpretedin a like fashion (e.g., “between” versus “directly between”, “adjacent”versus “directly adjacent”, etc.). Similarly, a term such as“communicatively connected” includes all variations of informationexchange routes between two devices, including intermediary devices,networks, etc., connected wirelessly or not.

As used herein, the singular forms “a”, “an” and “the” are intended toinclude both the singular and plural forms, unless the languageexplicitly indicates otherwise with words like “only,” “single,” and/or“one.” It will be further understood that the terms “comprises”,“comprising,”, “includes” and/or “including”, when used herein, specifythe presence of stated features, steps, operations, elements, ideas,and/or components, but do not themselves preclude the presence oraddition of one or more other features, steps, operations, elements,components, ideas, and/or groups thereof.

It should also be noted that the structures and operations discussedbelow may occur out of the order described and/or noted in the figures.For example, two operations and/or figures shown in succession may infact be executed concurrently or may sometimes be executed in thereverse order, depending upon the functionality/acts involved.Similarly, individual operations within example methods described belowmay be executed repetitively, individually or sequentially, so as toprovide looping or other series of operations aside from the singleoperations described below. It should be presumed that any embodimenthaving features and functionality described below, in any workablecombination, falls within the scope of example embodiments.

Applicants have recognized problems existing in several diverse types ofnuclear fuel cores with control element placement near certain fuelbundles. Particularly, Applicants have identified that while amaximization of fresh fuel within a nuclear fuel core at any beginningof cycle will permit longer cycle operating times and reduce outageintervals, such maximization can also force fresh fuel bundles to beplaced directly next to control elements, which may cause severalproblems over the life of the fuel, including corrosion, channel-bladeinterference, and pellet-cladding interactions. Applicants have furtherrecognized that Control Cell Core management techniques, where freshfuel bundles are not placed directly adjacent to control elements,restricts the amount of fresh fuel that can be placed within a core, aswell as limiting placement of fresh fuel in optimal positions for powermanagement, resulting in worsened burnup/efficiency and shorteroperating cycles. Example embodiments and methods below uniquely addressthese and other problems identified by Applicants in related nuclearfuel management technologies for a diverse array of nuclear plants.

Example Embodiments

Example embodiments of the invention include nuclear fuel cores havinghigher reactivity fuel in lower proportions adjacent to controlelements. Lower reactivity fuel is placed in greater proportionsadjacent to control elements, while permitting overall fuel content andoperating lifespan of the core to be substantially maintained. Exampleembodiments form two or more different types of positions subject todirect control element exposure—a larger number of controlled positionsof a first type having a higher population of burnt and/orlower-enrichment fuel; and a smaller number of controlled positions of asecond type having a higher population of fresh and/or higher-enrichmentfuel. Specific example embodiments describing how this configuration maybe achieved across several different core designs are discussed below,with the understanding that specific placements of the differing typesof controlled positions within various regions in example embodimentscan be varied based on core design and reactivity needs. It is furtherunderstood that any specific plant type, fuel type, enrichment level,exposure level, and/or control element configuration discussed in theseexample embodiments are not limiting but merely examples of the breadthof nuclear reactor technologies across which example embodiments may beimplemented. Example methods of forming and using example embodimentsare described thereafter.

FIG. 3 is a quadrant map of an example embodiment fuel core 100; forexample, FIG. 3 may be an initial loading map for a particular cycle.Core 100 may be useable in existing boiling water reactors; for example,core 100 may be useable in similar plants as the related fuel coreloading strategy of FIG. 2. As shown in FIG. 3, core 100 may include atypical BWR fuel core geometry, such as a 17-bundle radius quadrant.

Example embodiment core 100 can be visualized in three regions: an outerperiphery 120; an inner periphery 130; and an inner core 140. Outerperiphery 120 may be up to three fuel bundles thick from an edge of thecore in a reactor and include mostly burnt fuel bundles 111 (no fill).Burnt fuel bundles 111 are bundles that have experienced burnup inprevious operating cycles or otherwise have been exposed to neutron fluxor have significantly lower reactivity than fresh fuel bundles.

Inner periphery 130 may be up to three fuel bundles thick and include alarger proportion of higher enrichment fresh fuel bundles 110(cross-hatched fill). Inner core 140 includes the remainder of the corewithin inner periphery 130 and includes a mix of lower enrichment freshfuel bundles 112 (diagonal fill) and burnt fuel bundles 111. Fresh fuelbundles 110 and 112 may have little or no previous neutron flux exposurecompared to burnt fuel bundles 111. For example, fresh fuel bundles 110and 112 may be newly-manufactured bundles previously unused in coreoperations. Higher enrichment fresh fuel bundles 110 and lowerenrichment fresh fuel bundles 112 may differ in fissile materialenrichment by any degree required for core 100 operations andoptimization. For example, higher enrichment fresh fuel bundles 110 maycontain 4.3% Uranium-235 fuel, and lower enrichment fresh fuel bundles112 may include approximately 4.2% Uranium-235 fuel. Fuel bundles 110and 112 may each have distinct distributions and concentrations ofburnable absorber as well.

In other example embodiments, burnt fuel bundles 111, higher enrichmentfresh fuel bundles 110, and lower enrichment fresh fuel bundles 112 maybe replaced with fuel bundles having a same age but varying initialenrichment and burnable absorber concentration in order to achieve thesame reactivity differences as between bundles 110, 111, and 112 inexample embodiment core 100. Similarly, reactivity differences may beachieved by using bundles of a same initial enrichment but having threedifferent operating exposure levels, such as fresh, burnt 1-cycle, orburnt 2-cycles in place of higher enrichment fresh fuel bundles 110,lower enrichment fresh fuel bundles 112, and burnt fuel bundles 111. Yetfurther, reactivity and enrichment differences between all fresh fuelbundles 110 and 112 may be non-existent or minimal, such as where asingle fuel type and enrichment is used throughout an entire examplecore having only differently-aged fuel bundles.

Comparing FIGS. 2 and 3, it can be seen that example embodiment core 100includes more fresh fuel bundles in inner periphery region 130 and doesnot adhere to a strict checkerboard pattern for fresh and stale fuelbundles in inner core 140. In this way, example embodiment core 100 mayinclude substantially the same amount of fresh fuel bundles 110 and 112and/or fissile mass and reactivity as related art cores loaded formaximum operation cycle length. Instead of a strict checkerboardalteration between burnt fuel bundles 111 and lower enrichment freshbundles 112 in inner core 140, example embodiment core 100 includes somegroupings of fuel bundles that include more burnt bundles 111. As seenin FIG. 3, four burnt fuel bundles 111 may be grouped about a controlblade so as to form a priority control cell 142 that includes less freshfuel bundles than non-priority control cells 141. For example, as shownin FIG. 3, priority control cells 142, shown by a solid black linesurrounding bundle locations so controlled, may include only burnt fuelbundles 111. Priority control cells 142 may be inner-most control cellswithin inner region 140. Non-priority control cells 141, shown by abroken black line surrounding bundle locations so controlled, mayinclude a mix of burnt fuel bundles 111 and fresh bundles 112 similar torelated art core of FIG. 1 and may be positioned closer to or in innerperiphery 130, outside of priority control cells 142.

FIG. 4 is a quadrant map of an example embodiment fuel core 200; forexample, FIG. 4 may be an initial loading map for a particular cycle.Core 200 in the example of FIG. 4 may be useable in an EconomicSimplified Boiling Water Reactor (ESBWR). As shown in FIG. 4, core 200may include a typical ESBWR fuel core geometry, such as a 19-bundleradius quadrant.

Example embodiment core 200 can be visualized in three regions: an outerperiphery 220; an inner periphery 230; and an inner core 240. Outerperiphery 220 may be up to three fuel bundles thick from an edge of thecore in a reactor and include mostly once-burnt fuel bundles 213 (dashedfill) and twice-burnt fuel bundles 211 (no fill). Burnt fuel bundles 211and 213 are bundles that have experienced burnup in previous operatingcycles or otherwise have been exposed to neutron flux or havesignificantly lower reactivity than fresh fuel bundles. For example,once-burnt fuel bundles 213 have approximately 15-23 GWd/ST exposurefrom a single two-year operating cycle in known ESBWR cores, andtwice-burnt fuel bundles 211 may have more burnup, such as 35-40 GWd/STexposure.

Inner periphery 230 may be one to three fuel bundles thick and include alarger proportion of higher enrichment fresh fuel bundles 210(cross-hatched fill). Inner core 240 includes the remainder of the corewithin inner periphery 230 and includes a mix of mostly lower enrichmentfresh fuel bundles 212 (diagonal fill) and once-burnt fuel bundles 213.Fresh fuel bundles 210 and 212 may have little or no previous neutronflux exposure compared to burnt fuel bundles 211 and 213. For example,fresh fuel bundles 210 and 212 may be newly-manufactured bundlespreviously unused in core operations. Higher enrichment fresh fuelbundles 210 and lower enrichment fresh fuel bundles 212 may differ infissile material enrichment by any degree required for core 200operations and optimization. For example, higher enrichment fresh fuelbundles 210 may contain 4.3% Uranium-235 fuel, and lower enrichmentfresh fuel bundles 212 may include approximately 4.2% Uranium-235 fuel.Fuel bundles 210 and 212 may each have distinct distributions andconcentrations of burnable absorber as well.

In other example embodiments, twice-burnt fuel bundles 211, once-burntfuel bundles 213, higher enrichment fresh fuel bundles 210, and lowerenrichment fresh fuel bundles 212 may be replaced with fuel bundleshaving a same age but varying initial enrichment and burnable absorberconcentration in order to achieve the same reactivity differences asbetween bundles 210, 211, 212, and 213 in example embodiment core 200.Similarly, reactivity differences may be achieved by using bundles of asame initial enrichment but having three different operating exposurelevels, such as fresh, burnt 1-cycle, or burnt 2-cycles in place ofhigher enrichment fresh fuel bundles 210, lower enrichment fresh fuelbundles 212, and burnt fuel bundles 211 and 213. Yet further, reactivityand enrichment differences between fresh fuel bundles 210 and 212 may benon-existent or minimal, such as where a single fuel type and enrichmentis used throughout an entire example core having only differently-agedfuel bundles.

Example embodiment core 200 may include substantially the same amount offresh fuel bundles 210 and 212 and/or fissile mass as related art ESBWRcores loaded for maximum operation cycle length. Example embodiment core200 includes some groupings of fuel bundles that include more burntbundles 211 and/or 213. As seen in FIG. 4, four once-burnt fuel bundles213 may be grouped about a control blade so as to form a prioritycontrol cell 242 that includes less fresh fuel bundles and lessreactivity than non-priority control cells 241. For example, as shown inFIG. 4, priority control cells 242, shown by a solid black linesurrounding bundle locations so controlled, may include only once-burntfuel bundles 213. Priority control cells 242 may be inner-most controlcells within inner region 240. Non-priority control cells 241, shown bya broken black line surrounding bundle locations so controlled, mayinclude a mix of burnt fuel bundles 211 and 213 and fresh bundles 112and may be positioned closer to or in inner periphery 230.

Example embodiment cores are useable with fuel assemblies described inco-owned application Ser. No. 12/843,037 filed Jul. 25, 2010 titled“OPTIMIZED FUEL ASSEMBLY CHANNELS AND METHODS OF CREATING THE SAME,”which is incorporated herein by reference in its entirety. For example,fuel bundles that are to be placed in controlled positions in exampleembodiment cores may use channels with Zircaloy-4 to additionally guardagainst shadow corrosion.

Other example embodiment cores may be useable in Advanced Boiling WaterReactors, other Light and Heavy Water Reactors, or any nuclear reactorhaving nuclear chain reaction control structures extending into the corethat are useable to control reactivity, with modifications of size andinitial enrichments made for the appropriate type of core and controlelement placement.

Example Methods

Example methods include loading and/or operating nuclear cores. Examplemethods may take particular advantage of nuclear cores loaded asdescribed above in example embodiments, but it is understood thatexample methods and embodiments may be used separately.

During an operating outage or other time when a core is available forloading, an operator or other party may load a core so as to achieveloading patterns consistent with those described in the above exampleembodiments. For example, existing fuel bundles may be shuffled intostale fuel positions based on their age, enrichment, and/or reactivity.Such shuffling may open a number of positions about an inner peripheryand non-primary controlled locations within the inner core. A desirednumber of oldest or least functional fuel bundles may be removed fromthe core. Fresh fuel bundles may be procured and installed in locationsvacated by the fuel shuffle, based on enrichment or other parameters.Such shuffling may create a fuel core resembling example embodimentsdescribed above or related embodiments.

During operation of a core, control elements may be used to control thenuclear chain reaction. For example, in related art BWRs, a cruciformcontrol blade may be extended between four adjacent bundles in a controlcell to control reactivity. Example methods include using only controlelements directly adjacent to fuel bundles having relatively lowerreactivity and/or being previously burnt and not fresh for fine,day-to-day reactivity control within a core. In example methods, controlelements directly adjacent to fresh or higher reactivity fuel bundlesare relatively stationary and used for only coarse reactivityadjustments at a few set points during the fuel cycle; these controlelements may be entirely removed from the core—i.e., not used at all forreactivity control—during the later portions of the cycle.

As a specific example method in connection with the example embodimentof FIG. 4, an operator or other party may load an ESBWR core 200 so asto create priority control cells 242 including only burnt fuelassociated with control blades in a central area of inner core 240 ofcore 200. Non-priority control cells 241 including some fresh and/orhigh-reactivity fuel bundles are created at control blade positionsnearer or in the inner periphery region 230, outside of the prioritycontrol cells 242. During sequence exchange intervals occurring atapproximately every 3 GWd/ST of operation, for example, control bladesin non-priority cells 241 may be moved to a desired coarse reactivitycontrol position. Otherwise, control blades in non-priority cells 241are not required to be moved for reactivity control, and control bladesonly in priority control cells 242 may be moved for fine reactivitycontrol throughout the sequence. During the final quarter of operation,at approximately 15 GWd/ST cycle average exposure, for example, controlblades in non-priority cells 241 may be fully withdrawn and notnecessary to control reactivity. At all points during the cycle, controlblades associated with priority cells 242 may be freely moved to makefine adjustments to core reactivity. During the final quarter of thecycle, control blades in priority cells 242 alone may be used to controlcore reactivity; that is, control blades in priority cells 242 may bethe only blades within core 200 after approximately 15 GWd/ST.

Example embodiments and/or methods may provide fuel cores in existingand future-designed reactors with large enough fresh fuel reload batchsizes to accommodate longer operating cycles with higher powerdensities, while reducing or eliminating concerns associated withplacing fresh or higher reactivity fuel directly adjacent to controlelements. Placement of fresh fuel in greater numbers about an innerperiphery of the core and in limited number of controlled positions mayprovide a low-leakage core having several inner controlled positions notincluding fresh or high reactivity fuel. In this way, shadow corrosion,pellet-cladding interaction, and resulting channel distortions andnegative control element-channel interaction may be reduced by avoidingplacement of the newest and/or highest reactivity fuel bundles closestto active control elements. In addition to longer operating cyclecompatibility, high power density, lower leakage, and reduced channeldistortion, example embodiments and/or methods may permit nuclear fuelcores to be operated with simplified control element maneuvers;particularly, example embodiments and methods may permit only a subsetof control elements to be used for immediate, fine reactivity controland reduce a number of total control element sequences and exchangesthroughout an entire operating cycle and/or reduce any need to lowerpower during such complicated exchanges. These and other advantages andsolutions to newly-identified core operating problems are addressed bythe various example embodiments and methods described above.

Example embodiments and methods thus being described, it will beappreciated by one skilled in the art that example embodiments may bevaried and substituted through routine experimentation while stillfalling within the scope of the following claims. For example, a varietyof different nuclear fuel types and core designs are compatible withexample embodiments and methods simply through loading and operationalstrategy—and without any core geometry or structural changes—and fallwithin the scope of the claims. Such variations are not to be regardedas departure from the scope of these claims.

What is claimed is:
 1. A method of loading a nuclear reactor core havinga radius of at least seventeen fuel bundle positions, the methodcomprising: loading first control cells in a radial innermost area ofthe core with at least three burnt fuel bundles; loading second controlcells radially outside the first control cells with at least two freshfuel bundles per control cell; loading an outer region radially outsidethe second control cells with fresh fuel bundles; and loading anoutermost region radially outside the outer region with burnt fuelbundles.
 2. The method of claim 1, wherein the first control cellsoccupy all control cell positions in the radial innermost area of thecore and no first control cell is radially outside a second controlcell.
 3. The method of claim 2, wherein there are more first controlcells than second control cells, and wherein there are at least thirteenfirst control cells and at least twelve second control cells.
 4. Themethod of claim 3, wherein there are at least twenty-five first controlcells and at least twenty second control cells.
 5. The method of claim1, wherein the outer region and the outermost region are at least fourfuel bundles thick so that there are at least four fuel bundles betweenan edge of the core and all the second control cells.
 6. The method ofclaim 1, further comprising: loading at least two burnt fuel bundlesinto each of the second control cells.
 7. The method of claim 1, whereinthe only fresh fuel bundles in control cells in the core are in thesecond control cells.
 8. The method of claim 1, wherein the fresh fuelbundles have the highest reactivity of all the fuel bundles in the core,and wherein the fresh fuel bundles form a ring in the outer radialregion directly radially outside of the second control cells.
 9. Amethod of loading a nuclear reactor core having a radius of at leastseventeen fuel bundle positions, the method comprising: loading firstcontrol cells in a radial innermost area of the core with at least threefuel bundles of a first reactivity; loading second control cellsradially outside the first control cells with at least two fuel bundlesof a second reactivity per control cell; loading an outer regionradially outside the second control cells with fuel bundles of a thirdreactivity; and loading an outermost region radially outside the outerregion with fuel bundles of a fourth reactivity, wherein the firstreactivity is substantially lower than the second reactivity and thethird reactivity.
 10. The method of claim 9, wherein the firstreactivity is equivalent to approximately 15-23 GWd/ST exposure lowerthan the second reactivity.
 11. The method of claim 10, wherein thefourth reactivity is equivalent to 35-40 GWd/ST exposure lower than thesecond reactivity.
 12. The method of claim 9, wherein the first controlcells occupy all control cell positions in the radial innermost area ofthe core and no first control cell is radially outside a second controlcell.
 13. The method of claim 12, wherein there are more first controlcells than second control cells, and wherein there are at least thirteenfirst control cells and at least twelve second control cells.
 14. Themethod of claim 13, wherein there are at least twenty-five first controlcells and at least twenty second control cells.
 15. The method of claim9, wherein the outer region and the outermost region are at least fourfuel bundles thick so that there are at least four fuel bundles betweenan edge of the core and all the second control cells.
 16. The method ofclaim 1, further comprising: loading at least two fuel bundles of thefirst reactivity into each of the second control cells.
 17. The methodof claim 9, wherein the only fuel bundles of the second reactivity incontrol cells in the core are in the second control cells.
 18. Themethod of claim 1, wherein the second and third reactivity are thehighest reactivity of all the fuel bundles in the core, and wherein thefresh fuel bundles form a ring in the outer radial region directlyradially outside of the second control cells.